A conventional frequency division duplex (FDD) radio system simultaneously transmits a signal on one frequency band while receiving another signal on another frequency band. The context is illustrated in FIG. 2a-2b for communication between two radios.
FIG. 2a illustrates two radios: Radio-A 210 and Radio-B 230. Radios 210 and 230 each contain an associated transmitter 211, 231 and receiver 212, 232, respectively. If Radio-A 210 is upstream or closer to the central office station than Radio-B 230, then the signal transmitted from Radio-A 210 and received by Radio-B 230 is called the downlink (DL) signal 220. Similarly, in such a case, the signal transmitted from Radio-B 230 and received by Radio-A 210 is called the uplink (UL) signal 240. One illustrative context includes Radio-A 210 in a base station and Radio-B 230 in a mobile device, such as a mobile phone.
FIG. 2b illustrates key aspects of a frequency spectrum 250 in an FDD system such as drawn in FIG. 2a. The DL signal band 260 and UL signal band 280 occupy disjoint but not necessarily contiguous frequency bands. FIG. 2b illustrates the DL band 260 as being lower in frequency than the UL band 280, but those skilled in the art understand that that need not be the case in general.
Generally, there is also an inactive or unused portion of the spectrum between the DL band 260 and UL band 280 called the guard band (GB) 270. Because radio signals cannot be perfectly confined to a frequency band, i.e. they leak energy outside their prescribed band, the GB 270 serves to reduce the impact of leakage as the strongest leakage is typically closest to the signal's main band. However, as is known in the art, even with the use of a GB, the amount of isolation is finite and some degree of leakage always exists.
The basic elements of a radio 100 are shown in FIG. 1 and may apply to either Radio-A 210 or Radio-B 230 of FIG. 2A.
For the sake of clarity and without loss of generality, the remainder of the document will describe all radios as if they were the upstream Radio-A 210 that is transmitting a DL signal 220 and receiving an uplink signal 240, but it is obvious to those skilled in the art that the description could be equally applied to downstream Radio-B 230.
In the example of FIG. 1, the digital data is encoded into a baseband or low intermediate-frequency (IF) waveform and output in digital form from the baseband transmitter (BB Tx) 110 to a digital-to-analog converter (DAC) 120 which converts the digital signal into a continuous-time analog baseband or low IF waveform. The output of DAC 120 is then modulated up in frequency or up-converted by an up-converting mixer 130. The up-converting mixer 130 modulates the baseband output of DAC 120 up to the carrier frequency in the DL band 260 as set by local oscillator 135. The radio-frequency (RF) output of the up-converting mixer 130 is then amplified by one or more amplifiers including a dedicated power amplifier (PA) 140. The PA output drives one portion 151 of a duplexer 150 to an antenna 155. Filter 151 in the duplexer 150 serves to attenuate transmitted signal components outside the desired transmit or DL band 260 while maintaining the signal strength within the transmit or DL band 260. The antenna 155 radiates the transmit or DL signal 220 electromagnetically to another radio.
In the example of FIG. 1, the receiver portion of radio 100 works in a similar manner but in reverse. In particular, antenna 155 receives a radio signal 240 in the receive or UL band 280 and filters that with a filter 152 in the duplexer 150. Filter 152 in the duplexer 150 serves to attenuate signal components outside the receive or UL band 280 while preserving the strength of the signal within the receive or UL band 280. The output of filter 152 in the duplexer 150 is fed to a low-noise amplifier (LNA) 160 to amplify the signal. The output of LNA 160 is then down-converted from the receive or UL band 280 to baseband by the down-converting mixer 170 where the amount of frequency down-conversion is controlled by the frequency of local oscillator 175. The low-IF or baseband signal output by the down-converting mixer 170 is quantized by the receiver analog-to-digital converter 180 to form a digital representation of the baseband or low-IF signal which is then passed to the baseband receiver (BB Rx) 190 for demodulation and decoding.
Imperfect isolation in the duplexer filters 151, 152 and nonlinear characteristics of various radio components such as but not limited to the up-converting mixer 130, PA 140, duplexer 150, antenna 155, LNA 160, down-converting mixer 170, and ADC 180 can lead to the transmitted DL signal corrupting or reducing the signal fidelity of the received UL signal. Some of these impairments will be described with the aid of FIGS. 3-6 illustrating the signal spectrum at various points in the signal path.
FIG. 3 illustrates a signal spectrum 300 of the transmitted signal such as may be observed at the output of the PA 140.
In the example transmitted DL band 260, there reside a plurality (four are drawn but there may be more or fewer) of channels 310a-d carrying data at a prescribed transmit power level 320. An undesired, but practically unavoidable, artifact of the transmitted channels 310 are intermodulation (IM) products 340. The third order IM products 340a and 340b are closest in frequency to the DL band 260, followed by the fifth order IM products 340c and 340d, then the seventh order IM products 340e and 340f, and so on. The received UL band 280 will often overlap with the frequency of one or more of these IM products, and hence, the received UL signal 240 is subject to being distorted by these IM products. For that reason, communications standards such as CDMA, GSM, LTE, and others, will specify a minimum adjacent channel leakage ratio (ACLR) defining the ratio of the power 320 of the intended transmit DL signal to the maximum power 330 from any of the IM products 340 to limit the amount of distortion.
If the spectrum 300 in FIG. 3 were that of the PA 140 output, then the effect of the filter 151 in the duplexer 150 is illustrated in FIG. 4 showing the spectrum of the resulting transmit DL signal at the antenna 155.
In this example, the duplexer transmit filter 151 has little effect on the intended transmit signal 310 in the transmit DL band 260, but signal components outside the transmit DL band 260 are attenuated. Consequently, the IM products 340 have now been attenuated to lower levels shown by the smaller IM products 440a-f. 
Spectrum 500 in FIG. 5 similarly illustrates the signal spectrum of the received signal at the output of the duplexer receive filter 152.
In this example, the duplexer receiver filter 152 is configured to pass signals in the received UL band 280 unabated. Thus, the offending IM product in this band (drawn as the fifth IM product 440d) is unaffected along with the desired received signal 550 received off the air from the antenna 155. Other signal components outside the received UL band 280 have been attenuated in this example, such as the transmitted DL signal 510 and the other IM products 540a-c, e, f. 
Spectrum 600 in FIG. 6 illustrates the example signal as it may be after the LNA 160 or down-converting mixer 170, or ADC 180.
In this example, nonlinear behavior in any of these elements will tend to increase the levels of the transmitted signal IM products 640a-f due to intermodulation since the transmitted signal 510 is typically the strongest signal component even after all filtering in the duplexer 150. Of particular concern is that the level of the IM product in the receive UL band 280 may increase, and consequently, degrade the signal to interference ratio (a measure of signal fidelity) of the received signal 550 to the interferer 640d. 
As is well understood by those skilled in the art, radio systems (especially consumer mobile devices) are increasing their data capacity and consequently demanding more stringent signal fidelity. A common limitation in radio systems is the fidelity of the received signal from a distant device and, in particular, its corruption by leaked interference as described above. The present invention addresses this interference in multiple means to (i) reduce the leaked transmit blocker 510a-d in the receive path and (ii) reduce the offending IM product(s) 640 in the received UL band 280 from nonlinear components in the transmit and receive paths.